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Full text of "Text Book Of Mechanical Engineering"

21 o                                 Scraping.

pressure is put on with both hands, and the file carefully guided
in a perfectly horizontal direction, the position of the hands
being shewn in Fig. 221. Comparatively narrow surfaces that
are not to be scraped are generally smoothed by 'draw-filing,3 the
file teeth being rubbed with chalk to compel the small particles
to drop out, and thus avoid the scratching of the work, and a
still further polish given by means of fine emery cloth wrapped
round the file. The position is shewn at Fig. 222. There is
some difference in the grip of the file upon various materials, it
being greatest on wrought iron or steel, and least on cast iron or
brass, so that a file may best be used when new upon brass, then
on cast iron, and finally on wrought iron or steel, for it will grip
the latter when worn on the former; but the reverse method
would not be feasible. During filing the surface should be
constantly tested with straight edge, and when finishing, a hand
surface plate, being slightly greased with oil and red ochre, will,
on application to the \vork, at once indicate the parts to be taken
down, The skin of a casting should always be removed, either
by chipping or by pickling in dilute acid, before applying the file,
otherwise the teeth would be at once dulled by such a hard sur/ace.

Scraping.—If the surface is to be further trued, recourse is
had to the scraper. We will assume that the tool B, Fig. 203, is
to be used. It is held in the hand, as shewn in Fig. 223, and
the portions to be removed are discovered by smearing a hand
surface plate with oil and red ochre and applying the plate to the
work. Patches of colour will be transferred to' the higher por-
tions of the surface, and when these have been scraped down the
work is cleaned again and once more tried, when the colour
patches will be found larger in number, but smaller in size and
more evenly dispersed. The operation is continued until further
accuracy is hindered by the grain of tlie material. Then we have
what is known in the workshop as a trm plane.

Originating a Surface Plate.—When a new surface plate
is required it is generally topied from a standard plate kept in the
•workshop, the method of the last paragraph being employed-
But if no such standard be at hand, or if the truth of our first
pkte be doubted, it is necessary to use three plates in order to
originate a true surface. These three plates are first planed truly